The present invention concerns the treatment of dilute aqueous solutions of ethylene oxide containing dissolved gases in order to remove these gases from said solutions.
Such solutions can have various origins, but essentially originate from the synthesis of ethylene oxide by the catalytic oxidation of ethylene in the gaseous phase.
In such a synthesis, the ethylene oxide formed must be isolated from a gaseous mixture containing the ethylene oxide in a very dilute state and unconverted ethylene, unconverted oxygen, steam, carbon dioxide CO2, methane, ethane, nitrogen, rare gases and other impurities; the principal ones of which are aldehydes like formaldehyde and acetaldehyde.
The usual process to obtain the ethylene oxide from such a gaseous mixture comprises:
(a) Water absorption: The gaseous mixture originating from the reaction zone is placed in contact with water and a dilute aqueous solution of ethylene oxide is obtained containing about 2 to 3% of ethylene oxide by weight, impurities such as aldehydes and, in the dissolved state, particularly CO.sub.2, methane, ethane, ethylene, nitrogen, oxygen and rare gases;
(b) Desorption: The dilute ethylene oxide solution is subjected to a steam extraction in a column delivering at the bottom an aqueous stream practically devoid of ethylene oxide and at the top a gaseous mixture containing steam gases initially dissolved in the aqueous solution of ethylene oxide, and impurities such as aldehydes and about 30 to 60% by weight of ethylene oxide;
(c) Reabsorption: The preceding gaseous stream, previously cooled, is placed in contact with water, ethylene oxide is reabsorbed as well as CO.sub.2 and the major part of the impurities, whereas the greatest part of the gases initially present in the dissolved form with the ethylene oxide are separated in the form of a gaseous stream. The aqueous ethylene oxide solution resulting from the reabsorption stage generally contains between 5 and 15% of ethylene oxide by weight; and
(d) Distillation: The solution is then distilled in order to obtain pure ethylene oxide.
In addition, it is often necessary to insert a supplementary step between steps (c) and (d) in order to eliminate CO.sub.2. The thus decarbonated aqueous solution of ethylene oxide can be treated in order to make glycols, distilled in order to obtain purified ethylene oxide, or partly treated in order to make glycols and partly distilled.
The object of the invention is the treatment of aqueous solutions such as those resulting from steps (a) and (c) of a procedure for the synthesis of ethylene oxide and, more generally, a procedure for all dilute aqueous solutions of ethylene oxide containing, in the dissolved state, carbon dioxide and at least one other compound normally gaseous and present in the solutions of the type cited originating from an ethylene oxide manufacturing procedure, in order to eliminate the dissolved gases directly from these solutions, in such a way that after their separation these gases can advantageously be recycled in the synthesis of ethylene oxide without carrying with them any ethylene oxide.
In fact, if such is not the case, it is necessary to proceed with a supplementary step in order to separate the ethylene oxide from these gases, prior to being able to recycle them in the synthesis of ethylene oxide.
Such a separation obviously complicates the general process for isolating the ethylene oxide and is not commercially feasible.
This is, for instance, a problem with the technique described in U.S. Pat. No. 3,729,899 which consists of subjecting the dilute ethylene oxide solution to a flash vaporization. Another drawback of this technique is that only a partial separation results, at the most equal to 85% of the gases initially dissolved in the treated ethylene oxide solution.